Ships and boats can be immobilised by ropes fouling their propellers. In my British patent No. 2,204,549, I have described a line cutter having:
a rotatable cutting blade assembly mountable for rotation with the propeller shaft and including at least one rotatable cutting blade extending generally radially of the propeller shaft and having a cutting edge provided between an axially directed face and a circumferentially directed face; and
a stationary cutting blade assembly adapted to be restrained against rotation by cooperation with the shaft bearing and including a stationary cutting blade extending generally radially of the propeller shaft, having a cutting edge provided between an axially directed and a circumferentially directed face and arranged for cutting action of its cutting edge in conjunction with the cutting edge of the or each rotatable cutting blade on rotation of the propeller shaft with the axially directed faces passing opposite each other during such rotation;
wherein the cutting edge of the or each rotatable cutting blade and/or the cutting edge of the stationary cutting blade is/are provided with serrations, and the cutting edges are shaped for cutting action of radially inner ones of the serrations prior to such action of radially outer serrations on continued rotation of the propeller shaft.
Cavitation of propellers has been an acknowledged problem for many years. It occurs when the pressure on the forward face of the propeller blades drops below the vapour pressure at the ambient temperature for the occurrence of boiling of water in which the propeller is working. Vapour bubbles form in the water due to the pressure reduction. The pressure reduction is a transitory effect and the vapour bubbles implode, eroding the propeller. Propellers are designed to work close to the threshold of cavitation over their entire surface, which results in the maximum amount of thrust being developed by the propeller at its design speed. Rotational speeds have increased with high speed engines.
A line cutter disturbs the water flow to its propeller and reduces the water pressure locally. This aggravates cavitation of the propeller.